Method of forming a mask on surface

ABSTRACT

A method of forming a mask on a surface utilises the technique of drop on demand ejection to deposit selectively material on said surface. The method comprises the steps of depositing material on said surface to form an outline of the mask on the surface; and subsequently depositing material within said outline to form said mask. Alternatively, the method comprises the steps of depositing material on said surface to form an outline of those parts of the surface on which material is not to be deposited, and subsequently depositing material outside of said outline to form said mask. The formed outline has a greater height than the remainder of the mask so as to provide a barrier for controlling the flow of material subsequently deposited on the surface.

The present invention relates to a method of forming a mask on asurface. In one particular embodiment, the present invention relates tomethod of forming a solder mask, or resist, on a surface, such as acircuit board.

Known methods for forming a solder mask on a surface include screenprinting and photolithography.

Screen printing typically uses a patterned fabric screen havingperforations corresponding to those areas of the circuit board thatrequire resist protection. The screen is positioned over the circuitboard and a large amount of material is placed on the screen. A drawingrod is then pulled across the screen, and when the screen is lifted awayfrom the board a resist pattern is left on the board. This patterntypically has a uniform thickness and reasonably accurate definition.The thus deposited solder mask is then cured. Advantages of this methodare the speed at which a board can be covered and the high repeatabilityof the process. The disadvantages associated with screen printing arethe quantity of material wasted, the relatively long lead times, thecost of fabricating a screen and also the need to dispose of wastematerial. This method is also rather restricted at resolving featuresizes below 150 microns.

Photolithography involves fully covering a circuit board with aphotoresist (using, for example, dry film lamination, screen printing,curtain coating or spray coating) and subsequently exposing thephotoresist to UV light through a phototool, such as a contact mask, aproximity mask, or a projection mask, for selectively masking areas ofthe photoresist. After developing the exposed areas of the photoresist,the masked areas can be removed by etching to leave the desired featuresfor further processing. The advantages of this technique are theaccuracy of the final image and the speed at which boards can be coatedand patterned. The disadvantages however, are the need to chemicallydispose of waste material and the multi-stage subtractive process.

In a first aspect the present invention provides a method of forming asolder mask on a surface, the method utilising the technique of drop ondemand ejection to deposit selectively material on said surface, saidmethod comprising the steps of:

-   -   (a) depositing material on said surface to form an outline of        the mask on the surface; and    -   (b) subsequently depositing material within said outline to form        said mask.    -   wherein the formed outline has a greater height than the        remainder of the mask subsequently formed during step (b) so as        to provide a barrier for controlling the flow of material        deposited during step (b).

Preferably, step (a) further comprises exposing the deposited materialto electromagnetic radiation to effect curing of the deposited material.

The method preferably comprises, prior to step (b), repeating step (a)at least once so as to build up said outline from a plurality ofdepositions of material. The method preferably comprising repeating step(b) at least once, step (b) being repeated a number of times which isless than or equal to the number of times that step (a) is repeated.

Preferably, at least once step (b) comprises exposing the materialdeposited during that step to electromagnetic radiation to effect curingof that material. The time delay between completing deposition andexposing the deposited material to electromagnetic radiation may beselectively varied so as to control the surface finish of the depositedmask.

The remainder of the mask may be deposited so as to form regions oflocally greater height, for example, to protect certain features of thecircuit board during subsequent processing or to provide a thickerdielectric. This may be achieved by means of additional selectivedepositions of material during formation of the remainder of the mask orby locally depositing droplets of greater size using known “greyscale”technology.

At least part of the outline may defines those areas of the surface onwhich material is not to be deposited, material being deposited duringstep (b) outside of those areas. Thus, in a second aspect, the presentinvention provides a method of forming a solder mask on a surface, themethod utilising the technique of drop on demand ejection to depositselectively material on said surface, said method comprising the stepsof depositing material on said surface to form an outline of those areasof the surface on which material is not to be deposited, andsubsequently depositing material outside of the outline to form saidmask, wherein the formed outline has a greater height than the remainderof the mask subsequently formed on said surface so as to provide abarrier for controlling the flow of material deposited outside of theoutline. Features described above in relation to the first aspect mayequally be applied to the second aspect.

The electromagnetic radiation may comprise one of ultra violet and infrared radiation. The deposition material may comprise one of acrylic,silicone, polyimide, polytetrafluoroethylene, and epoxy.

It has been found by the inventors of the present invention that asuitable mask material may be deposited on a surface using acommercially available drop on demand inkjet printhead and preferablysubsequently cured using electromagnetic radiation to leave a soldermask covering the desired areas of the surface. The inventors have foundthat the present invention enables a mask to be formed quickly,accurately and at low cost with excellent control over the printedsurface coating. This method offers further advantages over conventionaltechnology in the effective use of ink, the reduction in waste from theprocess, removal of tooling costs and the versatility and flexibility ofdirect imaging.

The inventors have found that a plurality of steps are required to formthe mask accurately, in contrast to producing an image and ink jettingthe whole pattern in one step. To illustrate this point consider aconventional desktop printer where the user can produce a data file andoutput via a printer to paper in a single step. Most businesses uselaser printers instead of drop on demand or continuous jet printersbecause image quality is significantly better using laser printers. Thisdifference in quality is because of the wetting nature of fluid and thecapillary action paper exerts when wet. As a printer product thisresults in a blurred and dull image.

When printing a solder mask using a single deposition step, the sameeffect is seen, in that the image detail one wishes to preserve isblurred and inaccurate. One way of sustaining the image quality is bycuring the deposited image without delay, which in turn prevents thefluid from spreading and distorting the image features. However, whenthe image is laid down directly the finish to the coating isunattractive and non-uniform across the surface. So, in short there is acompromise between having a smooth finish and poor definition, and goodimage definition and poor surface finish.

The present invention provides two different deposition steps to enablea good image quality and a smooth finish to be achieved on thedeposition surface. The method involves firstly printing the outline ofany design features the user may want to include, and preferablyoverprinting the outline several times, such that the height of ink isbuilt up. The purpose of this is to create a dam, which provides abarrier to contain and control the flow of ink from flowing into areaswhere interconnections are made and need to be free of solder mask.Next, the dam contained can be filled and the soldermask allowed to flownaturally, thus giving a smooth finish before any final curing. FIG. 1illustrates an example of an image of outlines put down on the circuitboard first. As described above this is to control the natural flow ofthe fluid used when infilling the image. FIG. 2 illustrates the imagedeposited on the areas that need solder mask coverage. CAD/CAM softwarepackages are able to provide the two sets of images required with minorscripting.

EXAMPLE

To produce a solder mask on a surface specific parameters were used toset the print head parameters and curing rates. A single XaarJet™ XJ500printhead was set to a temperature of 40° C. and a supply voltage of35.5V. A curing lamp was positioned approximately 12 mm above thesubstrate during an initial cure. The ink to be deposited comprisedacrylate oligomers and isobornyl acrylate monomer (to reduce viscosity),UV curing agents and pigment. To initially cure this ink formulation aminimum of 15 mJ is required from a UV (Xenon) light source.

During the deposition of the outline pattern a deposition and curingspeed of 280 mm/s was used, this equates to curing between 0.05 and 2seconds, preferably approximately 0.5 s, after ink deposition. Three 720dpi outline layers were printed sequentially in this manner, leaving aclear outline of the pattern. To allow for interlacing, each layer wasformed from four discrete print passes, or depositions of material,using the single printhead. By using four printheads, a 720 dpi layermay be formed in a single pass.

During the second deposition stage (infill), two 720 dpi layers weredeposited to form the solder mask. Each pass making up the first layerwas cured without any delay between 0.05 and 2 seconds, preferablyapproximately 0.5 s, after deposition. For the second layer, no curingwas performed for the first and third passes respectively, curing beingperformed for the second and fourth passes approximately 10 s afterdeposition.

In summary, a method of forming a mask, preferably a solder mask, on asurface utilises the technique of drop on demand ejection to depositselectively material on said surface. The method comprises the steps ofdepositing material on said surface to form an outline of the mask onthe surface; and subsequently depositing material within said outline toform said mask. Alternatively, the method comprises the steps ofdepositing material on said surface to form an outline of those parts ofthe surface on which material is not to be deposited, and subsequentlydepositing material outside of said outline to form said mask. Theheight of the outline is greater than that of the remainder of the maskso as to provide a barrier for restricting the flow of materialsubsequently deposited on the surface.

It will be understood that the present invention has been describedabove purely by way of example, and modifications of detail can be madewithin the scope of the invention.

1-12. (canceled)
 13. A method of forming a mask on a surface, the methodutilising the technique of drop on demand ejection to depositselectively material on said surface, said method comprising the stepsof: (a) depositing material on said surface to form an outline of themask on the surface; and (b) subsequently depositing material withinsaid outline to form said mask, said outline providing a barrier forcontrolling the flow of said subsequently deposited material on saidsurface.
 14. A method according to claim 13, wherein step (b) comprisesfilling the outline formed during step (a).
 15. A method according toclam 13, wherein the formed outline has a greater height than theremainder of the mask subsequently formed during step (b).
 16. A methodaccording to clam 15, wherein step (a) further comprises exposing thedeposited material to electromagnetic radiation to effect curing of thedeposited material.
 17. A method according to claim 15, comprising,prior to step (b), repeating step (a) at least once so as to build upsaid outline from a plurality of depositions of material.
 18. A methodaccording to claim 17, comprising repeating step (b) at lest once, step(b) being repeated a number of times which is less than or equal to thenumber of times that step (a) is repeated.
 19. A method according toclaim 18, wherein at least once step (b) comprises exposing the materialdeposited during that step to electromagnetic radiation to effect curingof that material.
 20. A method according to claim 19, wherein the timedelay between completing deposition and exposing the deposited materialto electromagnetic radiation is selectively varied so as to control thesurface finish of the deposited mask.
 21. A method according to claim15, wherein the remainder of the mask is deposited so as to form regionsof locally greater height.
 22. A method according to claim 15 wherein atleast part of said outline defines those areas of the surface on whichmaterial is not to be deposited, material being deposited during step(b) outside of those areas.
 23. A method of forming a mask on a surface,the method utilising the technique of drop on demand ejection to depositselectively material on said surface, said method comprising the stepsof: depositing material on said surface to form an outline of thoseareas of the surface on which material is not to be deposited; andsubsequent depositing material outside of the outline to form said mask,said outline providing a barrier for controlling the flow of saidsubsequently deposited material on said surface.
 24. A method accordingto claim 23, wherein the formed outline has a greater height than theremainder of the mask subsequently formed during step (b).
 25. A methodaccording to claim 16, wherein said electromagnetic radiation comprisesone of ultra violet an infra red radiation.
 26. A method according toclaim 15, wherein said deposited material comprises one of acrylic,silicone, polyimide, polytetrafluoroethylene, and epoxy.
 27. A methodaccording to claim 13, wherein said mask comprises a 3-dimensionalstructure.
 28. A method according to claim 13, being a method forforming a solder mask.
 29. A method according to claim 13, wherein themask is a resist.
 30. A mask formed on a surface, said mask comprising:an outline formed by deposition of material on said surface; andmaterial subsequently deposited on said surface, said mask being formedutilising the technique of drop on demand ejection to depositselectively material on said surface, and said outline providing abarrier which controlled flow on said surface, during formation of saidmask, of said subsequently deposited material
 31. A mask according toclaim 30, wherein said subsequently deposited material has been allowedto flow freely to form said mask.
 32. A mask according to claim 30,wherein said outline has a greater height than said subsequentlydeposited material.
 33. A mask according to claim 30, being a threedimensional structure.
 34. A mask according to claim 30, being a soldermask.
 35. A mask according to claim 30, being a resist.